Introduce new Arroyo-specific syntax (watermarks and metadata)

[mwylde]

This PR introduces two new syntax elements for the ArroyoDialect, and also makes the meaningless STORED keyword optional for generated fields.

WATERMARK FOR

Allows users to configure the event-time column and watermark generation expression without needing to create and then reference virtual fields:

WATERMARK FOR fieldname [AS watermark_expr]

for example,

CREATE TABLE logs (
  timestamp TIMESTAMP NOT NULL,
  id INT,
  WATERMARK FOR timestamp AS timestamp - INTERVAL '5 seconds'
)

METADATA FROM

Allow users to configure a column to have metadata injected from the source:

field_name TYPE METADATA FROM 'key'

example:

CREATE TABLE logs (
    id TEXT,
    kafka_topic STRING METADATA FROM 'topic',
    log TEXT
)